The present invention relates to a method of forming a semiconductor device, and more particularly to a method of forming an interconnection in a contact hole formed in an insulation layer over a silicon substrate.
In recent years, high density and three-dimensional integration of the semiconductor device has been increasingly under development. A contact hole is formed in an insulation layer which extends over a silicon substrate. An interconnection is formed which extends over the insulation layer and within the contact hole so that the interconnection is connected through the contact hole to a semiconductor element formed on a surface of the silicon substrate. For the high density and three-dimensional integration, an aspect ratio of the contact hole is required to be high wherein the aspect ratio is defined as a ratio of a depth to an opening diameter of the contact hole. If an aluminum-based metal interconnection is formed in the high aspect ratio contact hole by the normal sputtering method, a disconnection of the aluminum-based metal interconnection may appear on side walls of the high aspect ratio contact hole.
In order to solve this problem, it was proposed that a tungsten interconnection be formed by a blanket chemical vapor deposition method in place of the above aluminum-based metal interconnection.
If the tungsten interconnection is formed by the blanket chemical vapor deposition method, it is required that the tungsten interconnection film be connected through a conductive barrier layer to a diffusion region formed on the surface of the silicon substrate for the purpose of suppression of the excess diffusion of tungsten into the diffusion layer. As the barrier layer, laminations of titanium films and titanium nitride films are available. If the titanium film is deposited by the normal sputtering method at room temperature, the titanium film formed on the side walls of the high aspect ratio contact hole is not a continuous film but includes large size titanium grains discontinuously, for which reason this tungsten film is likely to be peeled from the side walls of the contact hole by a subsequent heat treatment. It is thus difficult to fill the high aspect ratio contact hole with the titanium film completely.
In order to settle the above problem, the following method was proposed which is also disclosed in the Japanese laid-open patent publication No. 7-106281. The conventional method will be described with reference to FIGS. 1A through 1E.
With reference to FIG. 1A, field oxide films 202 are selectively formed on a surface on a silicon substrate 201 so that the field oxide films 202 surround an active region of the surface of the silicon substrate 201. A diffusion layer 203 is formed on the active region of the surface of the silicon substrate 201. An insulation film 204 having a thickness of 1 micrometer is deposited by a chemical vapor deposition method so that the insulation film 204 extends over the field oxide films 202 and the diffusion layer 203. A contact hole 205 is formed in the insulation film 204 so that the diffusion layer 203 is shown through the contact hole 205. The contact hole 205 has an opening diameter of 0.35 micrometers.
With reference to FIG. 1B, a titanium film 206 having a thickness of about 100 nanometers is deposited on the insulation film 204 and on the side walls of the contact hole 205 as well as on the diffusion layer 203 by a sputtering method at a substrate temperature in the range of 350.degree. C. to 450.degree. C. The titanium film 206 extends continuously not only over the insulation film 204 but also on the vertical side walls of the contact hole 205. This allows the tungsten film to be filled within the contact hole and prevents the tungsten film from being peeled from the side walls of the contact hole 205. The thickness of the titanium film 206 over the insulation film 204 is thick to secure the thickness of the titanium film 206 which covers the diffusion layer 203 at the bottom of the contact hole 205. The thickness of the titanium film 206 which covers the diffusion layer 203 is 4 nanometers.
With reference to FIG. 1C, when the silicon substrate 201 is held at room temperature, a titanium nitride film 207 having a thickness of 50 nanometers is formed. A conductive barrier film comprising laminations of a titanium film 206 and a titanium nitride film 207 has a thickness of 150 nanometers over the insulation film 204.
With reference to FIG. 1D, a lamp anneal is carried out at 700.degree. C. for 30 seconds to cause a titanium silicidation of the titanium film 206 having the thickness of about 4 nanometers covering the diffusion layer 203 to form a C49-structured titanium disilicide film 208 having a thickness of 12 nanometers.
With reference to FIG. 1E, a tungsten film 209 is entirely formed by the blanket chemical vapor deposition. Laminations of the tungsten film 209, the titanium nitride film 207 and the titanium film 206 are patterned to form interconnections.
The above conventional method has the following two problems. The titanium nitride film 207 formed under the above conditions and covering the diffusion layer 203 has a thickness of 2 nanometers, which is insufficient as a barrier layer. The titanium nitride film 207 is required to have a thickness of at least about 10 nanometers in order to prevent that the titanium film 206 reacts with a source gas WF.sub.6 at the bottom of the contact hole 203 during formation of the titanium film 206 whereby titanium fluoride and prevent that the tungsten film 209 is diffused through the titanium silicide film 208 into the diffusion layer 203. If the aspect ratio of the contact hole 205 is 2.85, then the minimum thickness of the titanium nitride film 207 over the insulation film 204 is about 250 nanometers. In order to secure the ohmic contact of the tungsten film 209 and the diffusion layer 203 at the bottom of the contact hole 205, at least 10 nanometers thickness of the titanium silicide film 208 is needed, for which reason the titanium film 206 covering the diffusion layer 203 at the bottom of the contact hole 205 is required to be at least 3.3 nanometers. If the aspect ratio of the contact hole 205 is about 2.85, then the minimum thickness of the titanium film 206 over the insulation layer 204 is about 83 nanometers. The conductive barrier film comprising the titanium film 206 and the titanium nitride film 207 at the bottom of the contact hole is required to have a thickness of at least 333 nanometers over the insulation film 204 for allowing the conductive barrier film to have the barrier and ohmic properties. If the conductive barrier film has a thickness of 300 nanometers or more over the insulation film 204 and the tungsten film 209 is formed on the conductive barrier film for subsequent patterning the same, then it is difficult to keep the pattern of the photo-resist because the etching rate of the conductive barrier film is not high with reference to that of the resist film. This makes it difficult to obtain desired interconnections. It is difficult to satisfy both the requirements for obtaining the barrier ability and for accurate patterning of the interconnection layers.
The second problem is concerned with the shape of the titanium nitride film 207. With reference to FIG. 1D, if the titanium nitride film 207 is formed by the above conventional method, then the titanium nitride film 207 has an overhang shape at the top edges of the contact hole 205 whereby due to shadowing effect, growth of the titanium nitride film 207 on the side walls and the bottom of the contact hole 205 is prevented. As a result, it is difficult to fill the contact hole 205 with the tungsten film 209 and in place a cavity 210 is formed in the contact hole 205 thereby resulting in deterioration of the electrical connection of the interconnection and the diffusion layer 203.
Collimated sputtering method is effective in order to solve the above first problem but not effective to the second problem.